Blowout preventers often are opened in the field to change the rams if a different pipe size is to be used and also to inspect the rams. The bolts used to secure bonnets to the blow-out preventer body are generally large and require a high torque for seating the bonnet on the body. These factors make the removal and replacement of blowout preventer bonnets a difficult and time consuming operation.
It has long been a practice to pivotally mount the bonnet, such bonnets then being secured to the body by a ring of bolts (Shaffer Tool Works, page 4564 et seq. 1962-63 Composite Catalog of Oil Field Equipment and Services, published by World Oil). This structure assists in the movement of the bonnets once the high torque bolts have been disconnected but the removal and replacement of the blowout preventer bonnets is still a difficult and time consuming operation.